Looks like we are sending the same signal in the working and non working case.
You can make absolutely sure of this by booting directly connected to TV (which works),
and then uplugging hdmi cable to go through amp (while kodi is running). Do you get a display?
Connect back to TV to confirm signal is still being sent).
If you see picture on TV before and after and no picture when using amp, I'd say you have some faulty
hardware outside of Pi. Check all cables. Try different cables. Try different sockets on amp and TV.
Try a different amp if available to try to narrow it down.